CHAPTER XIII. THE EVOLUTION OF THE PRIMATES.
BEFORE we proceed to study the past history of the Primates it is necessary to consider briefly the cardinal principles of phylogeny, for the neglect of some of them has been productive of no small amount of error in theories of evolution.
It is now generally believed that all animals are the collateral descendants of a common ancestor of simple structure, which lived at a very remote period in the history of the earth. In course of time its descendants changed their habits and diet, and came under the influence of different environments, with the result that their tissues became more or less modified to enable them to survive under the altered conditions. Some primitive features were retained, but some organs became greatly modified or " specialized." And it is important to discover the nature of anatomical characters before one can assess their true value for taxonomic purposes, or for setting the blood-relationships of animals.
There appear to be two reasons for the retention of primitive characters. In the first place they are useful to their possessors, or they would have been masked by adaptive modifications. In the second place the overdevelopment of one or more special senses, the adoption df sheltered modes of life, or the possession of a large